Luminescent materials that emit light in the visible range or the near infrared range and methods of forming thereof

ABSTRACT

Luminescent materials and methods of forming such materials arc described herein. In one embodiment, a luminescent material has the formula: [A a Sn b X x X′ x′ X″ x″ ][dopant], wherein A is included in the luminescent material as a monovalent cation; X, X′, and X″ are selected from fluorine, chlorine, bromine, and iodine; a is in the range of 1 to 5; b is in the range of 1 to 3; a sum of x, x′, and x″ is a +2 b;  and at least X′ is iodine, such that x′(a+2b)≧1/5.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/267,756, filed on Dec. 8, 2009, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to luminescent materials. Moreparticularly, the invention relates to luminescent materials that emitlight in the visible range or the near infrared range and methods offorming such materials.

BACKGROUND OF THE INVENTION

A solar module operates to convert energy from solar radiation intoelectricity, which is delivered to an external load to perform usefulwork. A solar module typically includes a set of photovoltaic (“PV”)cells, which can be connected in parallel, in series, or a combinationthereof The most common type of PV cell is a p-n junction device basedon crystalline silicon. Other types of PV cells can be based onamorphous silicon, polycrystalline silicon, germanium, organicmaterials, and Group III-V semiconductor materials, such as galliumarsenide.

During operation of an existing solar module, incident solar radiationpenetrates below a surface of the PV cell and is absorbed within the PVcell. A depth at which the solar radiation penetrates below the surfacecan depend upon an absorption coefficient of the PV cell. In the case ofa PV cell based on silicon, an absorption coefficient of silicon varieswith wavelength of solar radiation. For example, for solar radiation at900 nm, silicon has an absorption coefficient of about 100 cm⁻¹, and thesolar radiation can penetrate to a depth of about 100 μm. In contrast,for solar radiation at 450 nm, the absorption coefficient is greater atabout 10⁴ cm⁻¹, and the solar radiation can penetrate to a depth ofabout 1 μm. At a particular depth within the PV cell, absorption ofsolar radiation produces charge carriers in the form of electron-holepairs. Electrons exit the PV cell through one electrode, while holesexit the PV cell through another electrode. The net effect is a flow ofan electric current through the PV cell driven by incident solarradiation. The inability to convert the total incident solar radiationto useful electrical energy represents a loss or inefficiency of thesolar module.

Current solar modules typically suffer a number of technical limitationson the ability to efficiently convert incident solar radiation to usefulelectrical energy. One significant loss mechanism typically derives froma mismatch between an incident solar spectrum and an absorption spectrumof PV cells. In the case of a PV cell based on silicon, photons withenergy greater than a bandgap energy of silicon can lead to theproduction of photo-excited electron-hole pairs with excess energy. Suchexcess energy is typically not converted into electrical energy but israther typically lost as heat through hot charge carrier relaxation orthermalization. This heat can raise the temperature of the PV cell and,as result, can reduce the efficiency of the PV cell in terms of itsability to produce electron-hole pairs. In some instances, theefficiency of the PV cell can decrease by about 0.5 percent for every 1°C. rise in temperature. In conjunction with these thermalization losses,photons with energy less than the bandgap energy of silicon aretypically not absorbed and, thus, typically do not contribute to theconversion into electrical energy. As a result, a small range of theincident solar spectrum near the bandgap energy of silicon can beefficiently converted into useful electrical energy.

Also, in accordance with a junction design of a PV cell, chargeseparation of electron-hole pairs is typically confined to a depletionregion, which can be limited to a thickness of about 1 μm. Electron-holepairs that are produced further than a diffusion or drift length fromthe depletion region typically do not charge separate and, thus,typically do not contribute to the conversion into electrical energy.The depletion region is typically positioned within the PV cell at aparticular depth below a surface of the PV cell. The variation of theabsorption coefficient of silicon across an incident solar spectrum canimpose a compromise with respect to the depth and other characteristicsof the depletion region that reduces the efficiency of the PV cell. Forexample, while a particular depth of the depletion region can bedesirable for solar radiation at one wavelength, the same depth can beundesirable for solar radiation at a shorter wavelength. In particular,since the shorter wavelength solar radiation can penetrate below thesurface to a lesser degree, electron-hole pairs that are produced can betoo far from the depletion region to contribute to an electric current.

It is against this background that a need arose to develop theluminescent materials described herein.

SUMMARY OF THE INVENTION

Luminescent materials according to various embodiments of the inventioncan exhibit a number of desirable characteristics. In some embodiments,the luminescent materials can exhibit photoluminescence with a highquantum efficiency, with a narrow spectral width, and with a peakemission wavelength located within a desirable range of wavelengths,such as the visible range or the near infrared range. Also, thesephotoluminescent characteristics can be relatively insensitive over awide range of excitation wavelengths. The luminescent materials can haveother desirable characteristics, such as relating to their bandgapenergies and electrical conductivities. Advantageously, the luminescentmaterials can be inexpensively and readily formed for use in solarmodules and other applications.

In one embodiment, a luminescent material has the formula:

[A_(a)Sn_(b)X_(x)X′_(x′)X″_(x″)][dopant]

whereinA is included in the luminescent material as a monovalent cation;X, X′, and X″ are selected from fluorine, chlorine, bromine, and iodine;a is in the range of 1 to 5;b is in the range of 1 to 3;a sum of x, x′, and x″ is a+2b; andat least X′ is iodine, such that x′/(a+2b)≧⅕.

In another embodiment, a method of forming a luminescent materialincludes: (1) providing a source of A and X, wherein A is selected fromat least one of elements of Group IA, and X is selected from at leastone of elements of Group VIIB; (2) providing a source of B, wherein B isselected from at least one of elements of Group IVB; (3) subjecting thesource of A and X and the source of B to vacuum deposition to form a setof films adjacent to a substrate; and (4) heating the set of films to atemperature T_(heat) to form a luminescent material adjacent to thesubstrate, wherein the luminescent material includes A, B, and X, one ofthe source of A and X and the source of B has a lower melting pointT_(m1), another of the source of A and X and the source of B has ahigher melting point T_(m2), and T_(m1)<T_(heat)<T_(m2).

Other aspects and embodiments of the invention are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict the invention to any particular embodiment but aremerely meant to describe various embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof the invention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 illustrates normalized emission spectra of a set of luminescentmaterials, according to an embodiment of the invention.

FIG. 2 illustrates a perovskite-based microstructure of certainluminescent materials, according to an embodiment of the invention.

FIG. 3 illustrates X-ray diffraction data for UD930, according to anembodiment of the invention.

FIG. 4 illustrates a combined representation of an incident solarspectrum and measured absorption and emission spectra of UD930 inaccordance with an embodiment of the invention.

FIG. 5 through FIG. 8 illustrate manufacturing methods to formluminescent materials, according to some embodiments of the invention.

FIG. 9 illustrates a solar module implemented in accordance with anembodiment of the invention.

FIG. 10 illustrates measured photoluminescence intensity plotted as afunction of annealing temperature for UD930, according to an embodimentof the invention.

FIG. 11( a) illustrates excitation spectra for UD930 at temperatures inthe range of 12K to 300K, according to an embodiment of the invention.

FIG. 11( b) illustrates emission spectra for UD930 at temperatures inthe range of 12K to 300K, according to an embodiment of the invention.

DETAILED DESCRIPTION Definitions

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a luminescent material can include multipleluminescent materials unless the context clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or moreelements. Thus, for example, a set of layers can include a single layeror multiple layers. Elements of a set can also be referred to as membersof the set. Elements of a set can be the same or different. In someinstances, elements of a set can share one or more commoncharacteristics.

As used herein, the term “adjacent” refers to being near or adjoining.Adjacent elements can be spaced apart from one another or can be inactual or direct contact with one another. In some instances, adjacentelements can be connected to one another or can be formed integrallywith one another.

As used herein, the terms “substantially” and “substantial” refer to aconsiderable degree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels of the embodiments described herein.

As used herein, the terms “optional” and “optionally” mean that thesubsequently described event or circumstance may or may not occur andthat the description includes instances where the event or circumstanceoccurs and instances in which it does not.

As used herein, the term “size” refers to a characteristic dimension ofan object. Thus, for example, a size of an object that is spherical canrefer to a diameter of the object. In the case of an object that isnon-spherical, a size of the non-spherical object can refer to adiameter of a corresponding spherical object, where the correspondingspherical object exhibits or has a particular set of derivable ormeasurable characteristics that are substantially the same as those ofthe non-spherical object. Alternatively, or in conjunction, a size of anon-spherical object can refer to an average of various orthogonaldimensions of the object. Thus, for example, a size of an object that isa spheroidal can refer to an average of a major axis and a minor axis ofthe object. When referring to a set of objects as having a particularsize, it is contemplated that the objects can have a distribution ofsizes around the particular size. Thus, as used herein, a size of a setof objects can refer to a typical size of a distribution of sizes, suchas an average size, a median size, or a peak size.

As used herein, the term “sub-micron range” refers to a general range ofdimensions less than about 1μm or less than about 1,000 nm, such as lessthan about 999 nm, less than about 900 nm, less than about 800 nm, lessthan about 700 nm, less than about 600 nm, less than about 500 nm, lessthan about 400 nm, less than about 300 nm, or less than about 200 nm,and down to about 1 nm or less. In some instances, the term can refer toa particular sub-range within the general range, such as from about 1 nmto about 100 nm, from about 100 nm to about 200 nm, from about 200 nm toabout 300 nm, from about 300 nm to about 400 nm, from about 400 nm toabout 500 nm, from about 500 nm to about 600 nm, from about 600 nm toabout 700 nm, from about 700 nm to about 800 nm, from about 800 nm toabout 900 nm, or from about 900 nm to about 999 nm.

As used herein, the term “ultraviolet range” refers to a range ofwavelengths from about 5 nm to about 400 nm.

As used herein, the term “visible range” refers to a range ofwavelengths from about 400 nm to about 700 nm.

As used herein, the term “infrared range” refers to a range ofwavelengths from about 700 nm to about 2 mm. The infrared range includesthe “near infrared range,” which refers to a range of wavelengths fromabout 700 nm to about 5 μm, the “middle infrared range,” which refers toa range of wavelengths from about 5 nm to about 30 μm, and the “farinfrared range,” which refers to a range of wavelengths from about 30 μmto about 2 mm.

As used herein, the terms “reflection,” “reflect,” and “reflective”refer to a bending or a deflection of light, and the term “reflector”refers to an element that causes, induces, or is otherwise involved insuch bending or deflection. A bending or a deflection of light can besubstantially in a single direction, such as in the case of specularreflection, or can be in multiple directions, such as in the case ofdiffuse reflection or scattering. In general, light incident upon amaterial and light reflected from the material can have wavelengths thatare the same or different.

As used herein, the terms “luminescence,” “luminesce,” and “luminescent”refer to an emission of light in response to an energy excitation.Luminescence can occur based on relaxation from excited electronicstates of atoms or molecules and can include, for example,chemiluminescence, electroluminescence, photoluminescence,thermoluminescence, triboluminescence, and combinations thereof.Luminescence can also occur based on relaxation from excited states ofquasi-particles, such as excitons, bi-excitons, and exciton-polaritons.For example, in the case of photoluminescence, which can includefluorescence and phosphorescence, an excited state can be produced basedon a light excitation, such as absorption of light. In general, lightincident upon a material and light emitted by the material can havewavelengths that are the same or different.

As used herein, the term “optical quantum efficiency” or “OQE” refers toa ratio of the number of photons emitted by a photoluminescent materialto the number of photons absorbed by the photoluminescent material. Insome instances, an optical quantum efficiency can be represented as:OQE=η₁η₂, where η₁ corresponds to a fraction of absorbed photons leadingto the formation of excited states, such as excited states of excitons,and η₂ corresponds to an “internal quantum efficiency,” namely afraction of excited states undergoing radiative decay that yieldsemitted photons.

As used herein, the term “absorption spectrum” refers to arepresentation of absorption of light over a range of wavelengths. Insome instances, an absorption spectrum can refer to a plot of absorbance(or transmittance) of a material as a function of wavelength of lightincident upon the material.

As used herein, the term “emission spectrum” refers to a representationof emission of light over a range of wavelengths. In some instances, anemission spectrum can refer to a plot of intensity of light emitted by amaterial as a function of wavelength of the emitted light.

As used herein, the term “excitation spectrum” refers to anotherrepresentation of emission of light over a range of wavelengths. In someinstances, an excitation spectrum can refer to a plot of intensity oflight emitted by a material as a function of wavelength of lightincident upon the material.

As used herein, the term “Full Width at Half Maximum” or “FWHM” refersto a measure of spectral width. In some instances, a FWHM can refer to awidth of a spectrum at half of a peak intensity value.

As used herein with respect to a photoluminescent characteristic, theterm “substantially flat” refers to being substantially invariant withrespect to a change in wavelength. In some instances, a photoluminescentcharacteristic can be referred to as being substantially flat over arange of wavelengths if values of that characteristic within that rangeof wavelengths exhibit a standard deviation of less than 20 percent withrespect to an average value, such as less than 10 percent or less than 5percent.

As used herein with respect to an emission spectrum, the term“substantially monochromatic” refers to emission of light over a narrowrange of wavelengths. In some instances, an emission spectrum can bereferred to as being substantially monochromatic if a spectral width isno greater than 120 nm at FWHM, such as no greater than 100 nm at FWHM,no greater than 80 nm at FWHM, or no greater than 50 nm at FWHM.

As used herein, the term “dopant” refers to a chemical entity that ispresent in a material as an additive or an impurity. In some instances,the presence of a dopant in a material can alter a set ofcharacteristics of the material, such as its chemical, magnetic,electronic, or optical characteristics.

As used herein, the term “electron acceptor” refers to a chemical entitythat has a tendency to attract an electron from another chemical entity,while the term “electron donor” refers to a chemical entity that has atendency to provide an electron to another chemical entity. In someinstances, an electron acceptor can have a tendency to attract anelectron from an electron donor. It should be recognized that electronattracting and electron providing characteristics of a chemical entityare relative. In particular, a chemical entity that serves as anelectron acceptor in one instance can serve as an electron donor inanother instance. Examples of electron acceptors include positivelycharged chemical entities and chemical entities including atoms withrelatively high electronegativities. Examples of electron donors includenegatively charged chemical entities and chemical entities includingatoms with relatively low electronegativities.

A set of characteristics of a material can sometimes vary withtemperature. Unless otherwise specified herein, a characteristic of amaterial can be specified at room temperature, such as 300K or 27° C.

Luminescent Materials

Embodiments of the invention relate to luminescent materials having anumber of desirable characteristics. In particular, luminescentmaterials according to some embodiments of the invention can exhibitphotoluminescence with a high quantum efficiency, with a narrow spectralwidth, and with a peak emission wavelength located within a desirablerange of wavelengths. Also, these photoluminescent characteristics canbe relatively insensitive over a wide range of excitation wavelengths.Without being bound by a particular theory, these unusual and desirablecharacteristics can at least partly derive from a particularmicrostructure of the luminescent materials. Advantageously, theluminescent materials can be inexpensively and readily processed to forma variety of products, which, in turn, can be used in solar modules andother applications.

Desirable luminescent materials include a class of semiconductormaterials that can be represented with reference to the formula:

[A_(a)B_(b)X_(x)][dopants]  (1)

In formula (1), A is selected from elements of Group IA, such as sodium(e.g., as Na(I) or Na'), potassium (e.g., as K(I) or K⁺¹), rubidium(e.g., as Rb(I) or Rb⁺¹), and cesium (e.g., as Cs(I) or Cs⁺¹); B isselected from elements of Group VA, such as vanadium (e.g., as V(III) orV⁺³), elements of Group IB, such as copper (e.g., as Cu(I) or Cu⁺¹),silver (e.g., as Ag(I) or Ag⁺¹), and gold (e.g., as Au(I) or Au⁺¹),elements of Group IIB, such as zinc (e.g., as Zn(II) or Zn⁺²), cadmium(e.g., as Cd(II) or Cd⁺²), and mercury (e.g., as Hg(II) or Hg⁺²),elements of Group IIIB, such as gallium (e.g., as Ga(I) or Ga⁺¹), indium(e.g., as In(I) or In⁺¹), and thallium (e.g., as Tl(I) or Tl⁺¹),elements of Group IVB, such as germanium (e.g., as Ge(II) or Ge⁺² or asGe(IV) or Ge⁺⁴), tin (e.g., as Sn(II) or Sn⁺² or as Sn(IV) or Sn⁺⁴), andlead (e.g., as Pb(II) or Pb⁺² or as Pb(IV) or Pb⁺⁴), and elements ofGroup VB, such as bismuth (e.g., as Bi(III) or Bi⁺³); and X is selectedfrom elements of Group VIIB, such as fluorine (e.g., as F⁻¹), chlorine(e.g., as Cl), bromine (e.g., as Br⁻¹), and iodine (e.g., as I⁻¹). Stillreferring to formula (1), a is an integer that can be in the range of 1to 12, such as from 1 to 9 or from 1 to 5; b is an integer that can bein the range of 1 to 8, such as from 1 to 5 or from 1 to 3; and x is aninteger that can be in the range of 1 to 12, such as from 1 to 9 or from1 to 5. In some instances, x can be equal to a+2b, such as for purposesof charge balance when oxidation states of A, B, and X are +1, +2, and−1, respectively. For example, a can be equal to 1, and x can be equalto 1+2b. It is also contemplated that one or more of a, b, and x canhave fractional values within their respective ranges. It is alsocontemplated that X_(x) in formula (1) can be more generally representedas X_(x)X′_(x′) (or X_(x)X′_(x)X″_(x″)), where X and X′ (or X, X′, andX″) can be independently selected from elements of Group VIIB, and thesum of x and x′ (or the sum of x, x′, and x″) can be in the range of 1to 12, such as from 1 to 9 or from 1 to 5. With reference to thegeneralized version of formula (1), the sum of x and x′ (or the sum ofx, x′, and x″) can be equal to a+2b. For example, a can be equal to 1,and the sum of x and x′ (or the sum of x, x′, and x″) can be equal to1+2b. It is further contemplated that a blend or a mixture of differentluminescent materials represented by formula (1) can be used. Dopantscan be optionally included in a luminescent material represented byformula (1), and can be present in amounts that are less than about 5percent, such as less than about 1 percent or from about 0.1 percent toabout 1 percent, in terms of atomic percent or elemental composition.The dopants can derive from reactants that are used to form theluminescent material, or can derive from moisture, atmospheric gases, orother chemical entities present during the formation of the luminescentmaterial. In particular, the dopants can include cations, anions, orboth, which can form electron acceptor/electron donor pairs that aredispersed within a microstructure of the luminescent material.

Examples of luminescent materials represented by formula (1) includethose represented with reference to the formula:

[A_(a)Sn_(b)X_(x)][dopants]  (2)

In formula (2), A is selected from potassium, rubidium, and cesium; andX is selected from chlorine, bromine, and iodine. Still referring toformula (2), x can be equal to a+2b. In some instances, a can be equalto 1, and x can be equal to 1+2b. Several luminescent materials withdesirable characteristics can be represented as CsSnX₃[dopants] andinclude materials designated as UD700 and UD930. In the case of UD700, Xis bromine, and, in the case of UD930, X is iodine. UD700 exhibits apeak emission wavelength at about 695 nm, while UD930 exhibits a peakemission wavelength at about 950 nm. The spectral width of UD700 andUD930 is narrow (e.g., about 50 meV or less at FWHM), and the absorptionspectrum is substantially flat from the absorption edge into the farultraviolet. Photoluminescent emission of UD700 and UD930 is stimulatedby a wide range of wavelengths of solar radiation up to the absorptionedge of these materials at about 695 nm for UD700 and about 950 nm forUD930. The chloride analog, namely CsSnCl₃[dopants], exhibits a peakemission wavelength at about 450 nm, and can be desirable for certainimplementations. Normalized emission spectra of UD700, UD930, and thechloride analog, as measured using a xenon lamp source at about 300K,are illustrated in FIG. 1 in accordance with an embodiment of theinvention. Other luminescent materials with desirable characteristicsinclude CsSn₂X₅[dopants], Cs₂SnX₄[dopants], and CsSn₃X₇[dopants],mixtures thereof with, or without, CsSnX₃[dopants], such as a mixture ofCsSnX₃[dopants], CsSn₂X₅[dopants], and Cs₂SnX₄[dopants], and luminescentmaterials in which at least a fraction of cesium is substituted withanother monovalent ion of comparable size, such as CH₃NH₃ ⁺ or otherpoly-elemental, monovalent ions. Additional luminescent materials withdesirable characteristics include RbSnX₃[dopants], such asRbSnI₃[dopants] that exhibits a peak emission wavelength at about 705nm, and RbSnBr₃[dopants] that exhibits a peak emission wavelength atabout 540 nm. Further luminescent materials with desirablecharacteristics include KSnX₃[dopants], such as KSnBr₃[dopants] thatexhibits a peak emission wavelength at about 465 nm. Each of theseluminescent materials can be deposited as a film in a single layer or inmultiple layers interspersed with other layers formed from the sameluminescent material or different luminescent materials.

Additional examples of luminescent materials represented by formula (1)include those represented with reference to the formula:

[A_(a)Ge_(b)X_(x)][dopants]  (3)

In formula (3), A is selected from potassium, rubidium, and cesium; andX is selected from chlorine, bromine, and iodine. Still referring toformula (3), x can be equal to a+2b. In some instances, a can be equalto 1, and x can be equal to 1+2b. In the case that A is cesium, and X isiodine, for example, a luminescent material can sometimes be representedwith reference to the formula:

[CsGeI₃][dopants]  (4)

Additional examples of luminescent materials represented by formula (1)include those represented with reference to the formula:

[A_(a)Pb_(b)X_(x)][dopants]  (5)

In formula (5), A is selected from potassium, rubidium, and cesium; andX is selected from chlorine, bromine, and iodine. Still referring toformula (5), x can be equal to a+2b. In some instances, a can be equalto 1, and x can be equal to 1+2b. In the case that A is cesium, and X isiodine, for example, a luminescent material can sometimes be representedwith reference to the formula:

[CsPbI₃][dopants]  (6)

Additional examples of luminescent materials represented by formula (1)include those represented with reference to the formula:

[A_(a)Sn_(b)X_(x)X′_(x′)][dopants]  (7)

In formula (7), A is selected from potassium, rubidium, and cesium; andX and X′ are different and are selected from fluorine, chlorine,bromine, and iodine. Still referring to formula (7), the sum of x and x′can be equal to a+2b. In order to achieve desirable photoluminescentcharacteristics, at least one of X and X′ can be iodine, which canconstitute at least ⅕,at least ¼, at least ⅓,at least ½,or at least ⅔ ofa total number of halide ions. For example, in the case that X′ isiodine, x′|(a+2b)≧⅕, ≧¼, ≧⅓, ≧½, or ≧⅔. In some instances, a can beequal to 1, and the sum of x and x′ can be equal to 1+2b. In the casethat A is cesium, X is chlorine, and X′ is iodine, for example, aluminescent material can sometimes be represented with reference to oneof the formulas:

[CsSnClI₂][dopants]  (8)

[CsSnCl₂I][dopants]  (9)

[CsSn₂Cl₂I₃][dopants]  (10)

[CsSn₂Cl₃I₂][dopants]  (11)

[CsSn₂ClI₄][dopants]  (12)

[CsSn₂Cl₄I][dopants]  (13)

[Cs₂SnClI₃][dopants]  (14)

[Cs₂SnCl₂I₂][dopants]  (15)

[Cs₂SnCl₃I][dopants]  (16)

And, in the case that A is cesium, X is bromine, and X′ is iodine, forexample, a luminescent material can sometimes be represented withreference to one of the formulas:

[CsSnBrI₂][dopants]  (17)

[CsSnBr₂I][dopants]  (18)

[CsSn₂Br₂I₃][dopants]  (19)

[CsSn₂Br₃I₂][dopants]  (20)

[CsSn₂BrI₄][dopants]  (21)

[CsSn₂Br₄I][dopants]  (22)

[Cs₂SnBrI₃][dopants]  (23)

[Cs₂SnBr₂I₂][dopants]  (24)

[Cs₂SnBr₃I][dopants]  (25)

And, in the case that A is cesium, X is fluorine, and X′ is iodine, forexample, a luminescent material can sometimes be represented withreference to one of the formulas:

[CsSnFI₂][dopants]  (26)

[CsSnF₂I][dopants]  (27)

[CsSn₂F₂I₃][dopants]  (28)

[CsSn₂F₃I₂][dopants]  (29)

[CsSn₂FI₄][dopants]  (30)

[CsSn₂F₄I][dopants]  (31)

[Cs₂SnFI₃][dopants]  (32)

[Cs₂SnF₂I₂][dopants]  (33)

[Cs₂SnF₃I][dopants]  (34)

Further examples of luminescent materials represented by formula (1)include those represented with reference to the formula:

[A_(a)Sn_(b)X_(x)X′_(x′)X″_(x″)][dopants]  (35)

In formula (35), A is selected from potassium, rubidium, and cesium; andX, X′, and X″ are different and are selected from fluorine, chlorine,bromine, and iodine. Still referring to formula (35), the sum of x, x′,and x″ can be equal to a+2b. In order to achieve desirablephotoluminescent characteristics, at least one of X, X′, and X″ can beiodine, which can constitute at least ⅕,at least ¼,at least ⅓,at least½,or at least ⅔ of a total number of halide ions. For example, in thecase that X′ is iodine, x′/(a+2b)≧⅕, ≧¼, ≧⅓, ≧½, or ≧⅔. In someinstances, a can be equal to 1, and the sum of x, x′, and x″ can beequal to 1+2b. In the case that A is cesium, X is chlorine, X′ isiodine, and X″ is bromine, for example, a luminescent material cansometimes be represented with reference to one of the formulas:

[CsSnClIBr][dopants]  (36)

[CsSn₂ClI_(x′)Br_(4-x′)][dopants], x′=1, 2, or 3  (37)

[CsSn₂Cl₂I_(x′)Br_(3-x′)][dopants], x′=1 or 2  (38)

[CsSn₂Cl₃IBr][dopants]  (39)

[Cs₂SnClI_(x′)Br_(3-x′)][dopants], x′=1 or 2  (40)

[Cs₂SnCl₂IBr][dopants]  (41)

And, in the case that A is cesium, X is chlorine, X′ is iodine, and X″is fluorine, for example, a luminescent material can sometimes berepresented with reference to one of the formulas:

[CsSnClIF][dopants]  (42)

[CsSn₂ClI_(x′)F_(4-x′)][dopants], x′=1, 2, or 3  (43)

[CsSn₂Cl₂I_(x′)F_(3-x′)][dopants], x′=1 or 2  (44)

[CsSn₂Cl₃IF][dopants]  (45)

[Cs₂SnClI_(x′)F_(3-x′)][dopants], x′=1 or 2  (46)

[Cs₂SnCl₂IF][dopants]  (47)

Certain luminescent materials represented by formula (1) can have aperovskite-based microstructure. This perovskite-based microstructurecan be layered with relatively stronger chemical bonding within aparticular layer and relatively weaker chemical bonding betweendifferent layers. In particular, certain luminescent materialsrepresented by formula (1) can have a perovskite-based crystalstructure. This structure can be arranged in the form of a network ofBX₆ octahedral units along different planes, with B at the center ofeach octahedral unit and surrounded by X, and with A interstitialbetween the planes, as illustrated in FIG. 2 in accordance with anembodiment of the invention, where B is a cation, X is a monovalentanion, and A is a cation that serves to balance the total charge and tostabilize the crystal structure. Certain aspects of a perovskite-basedmicrostructure can be observed in X-ray diffraction (“XRD”) data, asillustrated in FIG. 3 for UD930 in accordance with an embodiment of theinvention.

Referring back to FIG. 2, dopants can be incorporated in aperovskite-based crystal structure, as manifested by, for example,substitution of a set of atoms included in the structure with a set ofdopants. In the case of UD930, for example, either, or both, Cs⁺¹ andSn⁺² can be substituted with a cation such as Sn(IV) or Sn⁺⁴, and I⁻¹can be substituted with an anion such as F⁻¹, Cl⁻¹, Br⁻¹, O⁻², OH⁻¹, orother anions with smaller radii relative to I⁻¹. The incorporation ofdopants can alter a perovskite-based crystal structure relative to theabsence of the dopants, as manifested by, for example, shorter bondlengths along a particular plane and between different planes, such asshorter B-X-B bond lengths along a particular plane and shorter B-X-Bbond lengths between different planes. In some instances, substitutionof I⁻¹ with either, or both, of F⁻¹ and Cl⁻¹ can lead to shorter andstronger bonds with respect to Sn⁺² along a particular plane and betweendifferent planes. Without being bound by a particular theory, theincorporation of dopants can lend greater stability to aperovskite-based crystal structure, and desirable photoluminescentcharacteristics can at least partly derive from the presence of thesedopants. In some instances, substitution of I⁻¹ with other halides canbe at levels greater than typical doping levels, such as up to about 50percent of I⁻¹ to form an alloy of mixed halides.

Certain luminescent materials represented by formula (1) can bepolycrystalline with constituent crystallites or grains having sizes inthe sub-micron range. The configuration of grains can vary from one thatis quasi-isotropic, namely in which the grains are relatively uniform inshape and size and exhibit a relatively uniform grain boundaryorientation, to one that is anisotropic, namely in which the grainsexhibit relatively large deviations in terms of shape, size, grainboundary orientation, texture, or a combination thereof. In the case ofUD930, for example, grains can be formed in an anisotropic fashion andwith an average size in the range of about 200 nm to about 400 nm, suchas from about 250 nm to about 350 nm.

Several luminescent materials represented by formula (1) havecharacteristics that are desirable for solar modules. In particular, theluminescent materials can exhibit photoluminescence with a high opticalquantum efficiency that is greater than about 6 percent, such as atleast about 10 percent, at least about 20 percent, at least about 25percent, at least about 30 percent, or at least about 35 percent, andcan be up to about 40 percent, about 50 percent, or more, and with ahigh internal quantum efficiency that is greater than about 50 percent,such as at least about 60 percent, at least about 70 percent, at leastabout 75 percent, at least about 80 percent, or at least about 85percent, and can be up to about 95 percent, about 99 percent, or more.Also, the luminescent materials can exhibit photoluminescence with anarrow spectral width that is no greater than about 120 nm at FWHM, suchas no greater than about 100 nm or no greater than about 80 nm at FWHM.Thus, for example, the spectral width can be in the range of about 20 nmto about 120 nm at FWHM, such as from about 50 nm to about 120 nm, fromabout 50 nm to about 100 nm, or from about 50 nm to about 80 nm at FWHM.Incorporation of the luminescent materials within a resonant cavitywaveguide can further narrow the spectral width.

In addition, the luminescent materials can have bandgap energies andresistivities that are tunable to desirable levels by adjustingreactants and processing conditions that are used. For example, abandgap energy can correlate with A, with the order of increasingbandgap energy corresponding to, for example, cesium, rubidium,potassium, and sodium. As another example, the bandgap energy cancorrelate with X, with the order of increasing bandgap energycorresponding to, for example, iodine, bromine, chlorine, and fluorine.This order of increasing bandgap energy can translate into an order ofdecreasing peak emission wavelength. Thus, for example, a luminescentmaterial including iodine can sometimes exhibit a peak emissionwavelength in the range of about 900 nm to about 1 μm, while aluminescent material including bromine or chlorine can sometimes exhibita peak emission wavelength in the range of about 700 nm to about 800 nm.By tuning bandgap energies, the resulting photoluminescence can have apeak emission wavelength located within a desirable range ofwavelengths, such as the visible range or the infrared range. In someinstances, the peak emission wavelength can be located in the nearinfrared range, such as from about 900 nm to about 1 μm, from about 910nm to about 1 μm, from about 910 nm to about 980 nm, or from about 930nm to about 980 nm.

Moreover, the photoluminescence characteristics described above can berelatively insensitive over a wide range of excitation wavelengths.Indeed, this unusual characteristic can be appreciated with reference toexcitation spectra of the luminescent materials, which excitationspectra can be substantially flat over a range of excitation wavelengthsencompassing portions of the ultraviolet range, the visible range, andthe infrared range. In some instances, the excitation spectra can besubstantially flat over a range of excitation wavelengths from about 200nm to about 1 μm, such as from about 200 nm to about 980 nm or fromabout 200 nm to about 950 nm. Similarly, absorption spectra of theluminescent materials can be substantially flat over a range ofexcitation wavelengths encompassing portions of the ultraviolet range,the visible range, and the infrared range. In some instances, theabsorption spectra can be substantially flat over a range of excitationwavelengths from about 200 nm to about 1 μm, such as from about 200 nmto about 980 nm or from about 200 nm to about 950 nm. Also, opticalquantum efficiencies of the luminescent materials can be substantiallyflat over a range of excitation wavelengths, such as from about 200 nmto about 1 μm, from about 200 nm to about 980 nm or from about 450 nm toabout 900 nm.

For example, UD930 has a direct bandgap with a value of about 1.32 eV at300K. This bandgap can decrease as temperature decreases, at leastpartly resulting from an anharmonicity in a lattice potential. Withoutbeing bound by a particular theory, photoluminescence for UD930 (andcertain other luminescent materials represented by formula (1)) canoccur via exciton emission. An exciton corresponds to an electron-holepair, which can be formed as a result of light absorption. Mostsemiconductor materials have relatively small exciton binding energies,so excitons are typically not present at room temperature. Certainluminescent materials represented by formula (1) can have relativelylarge exciton binding energies, and can be incorporated in a resonantcavity waveguide to yield suppression of emission in a verticaldirection and stimulated emission along a plane of the cavity waveguide.The larger a Stokes shift, or exciton binding energy, the more tolerantthe cavity waveguide can be with respect to imperfections. Thus, thecavity waveguide can be readily formed in an inexpensive manner, withoutresorting to techniques such as Molecular Beam Epitaxy (“MBE”).

Desirable characteristics of UD930 can be further appreciated withreference to FIG. 4, which illustrates a combined representation of asolar spectrum and measured absorption and emission spectra of UD930 inaccordance with an embodiment of the invention. In particular, FIG. 4illustrates the AM1.5G solar spectrum (referenced as (A)), which is astandard solar spectrum representing incident solar radiation on thesurface of the earth. The AM1.5G solar spectrum has a gap in the regionof 930 nm due to atmospheric absorption. In view of the AM1.5G solarspectrum and characteristics of PV cells based on silicon, theabsorption spectrum (referenced as (B)) and emission spectrum(referenced as (C)) of UD930 render this material particularly effectivefor spectral concentration when incorporated within an emission layer.In particular, photoluminescence of UD930 is substantially located inthe gap of the AM1.5G solar spectrum, with the peak emission wavelengthof about 950 nm falling within the gap. This, in turn, allows the use ofreflector layers (e.g., above and below the emission layer) that aretuned to reflect emitted radiation back towards the emission layer,without significant reduction of incident solar radiation that can passthrough the reflector layers and reach UD930. Also, the absorptionspectrum of UD930 is substantially flat and extends from the absorptionedge at about 950 nm through a large fraction of the AM1.5G solarspectrum into the ultraviolet. In addition, the peak emission wavelengthof about 950 nm (or about 1.32 eV) is matched to the absorption edge ofPV cells based on silicon, and the spectral width is about 50 meV atFWHM (or about 37 nm at FWHM). The absorption coefficient of silicon isabout 10² in this range of emission wavelengths, and junctions withinthe PV cells can be designed to efficiently absorb the emitted radiationand convert the radiation into electron-hole pairs. As a result, UD930can broadly absorb a wide range of wavelengths from incident solarradiation, while emitting a narrow range of wavelengths that are matchedto silicon to allow a high conversion efficiency of incident solarradiation into electricity. Furthermore, the absorption spectrum and theemission spectrum of UD930 overlap to a low degree, thereby reducinginstances of self-absorption that would otherwise lead to reducedconversion efficiency.

Luminescent materials represented by formula (1) can be formed viareaction of a set of reactants or precursors at high yields and atmoderate temperatures and pressures. The reaction can be representedwith reference to the formula:

Source(B)+Source(A, X)→Luminescent Material  (48)

In formula (48), source(B) serves as a source of B, and, in someinstances, source(B) can also serve as a source of dopants or halideions. In the case that B is germanium, tin, or lead, for example,source(B) can include one or more types of B-containing compoundsselected from B(II) compounds of the form BY, BY₂, B₃Y₂, and B₂Y andB(IV) compounds of the form BY₄, where Y can be selected from elementsof Group VIB, such as oxygen (e.g., as O⁻²); elements of Group VIIB,such as fluorine (e.g., as F⁻¹), chlorine (e.g., as Cl⁻¹), bromine(e.g., as Br⁻¹), and iodine (e.g., as F⁻¹); and poly-elemental chemicalentities, such as nitrate (i.e., NO₃ ⁻¹), thiocyanate (i.e., SCN⁻¹),hypochlorite (i.e., OCl⁻¹), sulfate (i.e., SO₄ ⁻²), orthophosphate(i.e., PO₄ ⁻³), metaphosphate (i.e., PO₃ ⁻¹), oxalate (i.e., C₂O₄ ⁻²),methanesulfonate (i.e., CH₃SO₃ ⁻¹), trifluoromethanesulfonate (i.e.,CF₃SO₃ ⁻¹), and pyrophosphate (i.e., P₂O₇ ⁻⁴). Examples of tin(II)compounds include tin(II) fluoride (i.e., SnF₂), tin(II) chloride (i.e.,SnCl₂), tin(II) chloride dihydrate (i.e., SnCl₂.2H₂O), tin(II) bromide(i.e., SnBr₂), tin(II) iodide (i.e., SnI₂), tin(II) oxide (i.e., SnO),tin(II) sulfate (i.e., SnSO₄), tin(II) orthophosphate (i.e., Sn₃(PO₄)₂),tin(II) metaphosphate (i.e., Sn(PO₃)₂), tin(II) oxalate (i.e.,Sn(C₂O₄)), tin(II) methanesulfonate (i.e., Sn(CH₃SO₃)₂), tin(II)pyrophosphate (i.e., Sn₂P₂O₇), and tin(II) trifluoromethanesulfonate(i.e., Sn(CF₃SO₃)₂). Examples of tin (IV) compounds include tin(IV)chloride (i.e., SnCl₄) and tin(IV) chloride pentahydrate (i.e.,SnCl₄.5H₂O). It is contemplated that different types of source(B) can beused, such as source(B) and source(B′), with B and B′ independentlyselected from elements of Group IVB, or as source(B), source(B′), andsource(B″), with B, B′, and B″ independently selected from elements ofGroup IVB.

Still referring to formula (48), source(A, X) serves as a source of Aand X, and, in some instances, source(A, X) can also serve as a sourceof dopants. Examples of source(A, X) include alkali halides of the formAX. In the case that A is cesium, potassium, or rubidium, for example,source(A, X) can include one or more types of A(I) halides, such ascesium(I) fluoride (i.e., CsF), cesium(I) chloride (i.e., CsCI),cesium(I) bromide (i.e., CsBr), cesium(I) iodide (i.e., CsI),potassium(I) fluoride (i.e., KF), potassium(I) chloride (i.e., KCl),potassium(I) bromide (i.e., KBr), potassium(I) iodide (i.e., KI),rubidium(I) fluoride (i.e., RbF), rubidium(I) chloride (i.e., RbCl),rubidium(I) bromide (i.e., RbBr), and rubidium(I) iodide (i.e., RbI). Itis contemplated that different types of source(A, X) can be used, suchas source(A, X) and source(A′, X′), with A and A′ independently selectedfrom elements of Group IA, and X and X′ independently selected fromelements of Group VIIB, or as source(A, X), source(A X′), and source(A′,X″), with A, A′, and A″ independently selected from elements of GroupIA, and X, X′, and X″ independently selected from elements of GroupVIIB.

The reaction represented by formula (48) can be carried out bycombining, mixing, or otherwise contacting source(B) with source(A, X),and then applying a form of energy. For some embodiments, source(B) andsource(A, X) can be deposited on a substrate to form a set of films orlayers. For example, source(B) and source(A, X) can be co-deposited on asubstrate to form a film, or can be sequentially deposited to formadjacent films. Examples of suitable deposition techniques includevacuum deposition (e.g., thermal evaporation or electron-beamevaporation), Physical Vapor Deposition (“PVD”), Chemical VaporDeposition (“CVD”), Atomic Layer Deposition (“ALD”), sputtering, spraycoating, dip coating, web coating, wet coating, and spin coating. Forother embodiments, source(B) and source(A, X) can be mixed in a dryform, in solution, or in accordance with any other suitable mixingtechnique. For example, source(B) and source(A, X) can be provided in apowdered form, and can be mixed using any suitable dry mixing technique.As another example, source(B) and source(A, X) can be dispersed in areaction medium to form a reaction mixture, and the reaction medium caninclude a solvent or a mixture of solvents. Once source(B) and source(A,X) are suitably combined, a form of energy is applied to promoteformation of a luminescent material, such as in the form of acoustic orvibrational energy, electrical energy, magnetic energy, mechanicalenergy, optical energy, or thermal energy. For example, source(B) andsource(A, A) can be deposited on a substrate, and a resulting set offilms can be heated to a suitable temperature to form the luminescentmaterial. Heating can be performed in air, in an inert atmosphere (e.g.,a nitrogen atmosphere), or in a reducing atmosphere for a suitable timeperiod. It is also contemplated that multiple forms of energy can beapplied simultaneously or sequentially.

The resulting luminescent material can include A, B, and X as majorelemental components as well as elemental components derived from orcorresponding to Y. Also, the luminescent material can includeadditional elemental components, such as carbon, chlorine, hydrogen, andoxygen, that can be present in amounts that are less than about 5percent or less than about 1 percent in terms of elemental composition,and further elemental components, such as sodium, sulfur, phosphorus,and potassium, that can be present in trace amounts that are less thanabout 0.1 percent in terms of elemental composition.

Examples of the reaction represented by formula (48) include thoserepresented with reference to the formula:

BY ₂ +AX→Luminescent Material  (49)

In formula (49), B is selected from germanium, tin, and lead; Y isselected from fluorine, chlorine, bromine, and iodine; A is selectedfrom potassium, rubidium, and cesium; and X is selected from fluorine,chlorine, bromine, and iodine. Still referring to formula (26), it iscontemplated that BY₂ can be more generally represented as BY₂ and B′Y′₂(or BY₂, B′Y′₂, and B″Y″₂), where B and B′ (or B, B′, and B″) areindependently selected from germanium, tin, and lead, and Y and Y′ (orY, Y′, and Y″) are independently selected from fluorine, chlorine,bromine, and iodine. In the case that B is tin, for example, BY₂ can berepresented as SnY₂, or can be more generally represented as SnY₂ andSnY′₂ (or SnY₂, SnY′₂, and SnY″₂), where Y and Y′ (or Y, Y′, and Y″) areindependently selected from fluorine, chlorine, bromine, and iodine.

For example, SnI₂ (or SnCl₂) can be reacted with CsI to form aluminescent material having a peak emission wavelength at about 950 nm,such as UD930. As another example, SnBr₂ can be reacted with CsBr toform a luminescent material having a peak emission wavelength at about695 nm, such as UD700. As another example, SnBr₂ can be reacted with KBrto form a luminescent material having a peak emission wavelength atabout 465 nm. As another example, SnI₂ can be reacted with RbI to form aluminescent material having a peak emission wavelength at about 705 nm.As a further example, SnBr₂ can be reacted with RbBr to form aluminescent material having a peak emission wavelength at about 540 nm.

Attention next turns to FIG. 5 through FIG. 8, which illustratemanufacturing methods to form luminescent materials, according to someembodiments of the invention.

Referring first to FIG. 5, a substrate 500 is provided. The substrate500 serves as a supporting structure during manufacturing operations,and serves to protect a resulting luminescent material fromenvironmental conditions. The substrate 500 can be rigid or flexible,can be porous or non-porous, can be optically transparent, translucent,or opaque, and can be formed from a glass, a metal, a ceramic, apolymer, or another suitable material. For some implementations, thesubstrate 500 can include a base substrate and a set of coatings orfilms that are formed adjacent to the base substrate to provide adeposition surface for subsequent manufacturing operations. Examples ofsuitable coating materials include oxides, such as silica (i.e., SiO₂ orα-SiO₂), alumina (i.e., Al₂O₃), TiO₂, Ta₂O₅, Nb₂O₅, ZrO₂, HfO₂, SnO₂,ZnO₂, La₂O₃, Y₂O₃, CeO₂, Sc₂O₃, Er₂O₃, V₂O₅, and In₂O₃; and othersuitable thin-film dielectric materials.

Substrate effects can sometimes occur with respect to resultingphotoluminescence characteristics. For example, enhancements of aboutthree times or more in photoluminescence intensity, such as from about 5times to about 10 times, can occur for alumina-based ceramic substrates,relative to substrates formed from certain other materials. Withoutbeing bound by a particular theory, such enhancements can at leastpartly derive from one or a combination of the following: a R1 R2emission process; surface roughness of the alumina-based ceramicsubstrate; and high reflectivity of the alumina-based ceramic substrate,which can promote reflection of incident radiation back towards aluminescent material.

Next, as illustrated in FIG. 5, a set of reactants (that are precursorsof a luminescent material) are deposited adjacent to the substrate 500.In the illustrated embodiment, source(B) and source(A, X) are subjectedto vacuum deposition, thereby forming a precursor layer 502 adjacent tothe substrate 500. Deposition can be carried out using a vacuumdeposition system that is evacuated to a pressure no greater than about1×10⁻⁴ Torr, such as no greater than about 1×10⁻⁵ Torr, and down toabout 1×10⁻⁶ Torr or less. It is contemplated that another suitabledeposition technique can be used in place of, or in conjunction with,vacuum deposition.

Deposition of source(B) and source(A, X) can be carried out sequentiallyin accordance with the same vacuum deposition technique or differentvacuum deposition techniques. For example, BY₂ and AX can be evaporatedin sequential layers, from two layers to 30 or more layers total, suchas from two layers to 16 layers total, or from two layers to six layerstotal, and with a weight or molar ratio of BY₂ to AX from about 99:1 toabout 1:99, such as from about 5:1 to about 1:5 or from about 2:1 toabout 1:2. A particular one of BY₂ and AX having a lower melting pointT_(m1) can be placed in an evaporator boat and deposited by thermalevaporation, while another one of BY₂ and AX having a higher meltingpoint T_(m2) can be placed in another evaporator boat and deposited bythermal evaporation or electron-beam evaporation. In the case of SnI₂with a melting point of about 318° C. (or SnCl₂ with a melting point ofabout 246° C.) and CsI with a melting point of about 620° C., SnI₂ (orSnCl₂) can be deposited by thermal evaporation, while CsI can bedeposited by thermal evaporation or electron-beam evaporation. Athickness of each individual BY₂-containing layer or each individualAX-containing layer can be in the range of about 10 nm to about 1.5 μm,such as from about 10 nm to about 1 μm or from about 10 nm to about 300nm, with a total thickness for all layers in the range of about 20 nm toabout 45 μm, such as from about 40 nm to about 20 μm or from about 50 nmto about 5 μm.

Source(B) and source(A, X) can also be co-deposited in accordance with aparticular vacuum deposition technique. For example, BY₂ and AX can beco-evaporated to form a single layer or multiple layers, with a weightor molar ratio of BY₂ to AX from about 10:1 to about 1:10, such as fromabout 5:1 to about 1:5 or from about 2:1 to about 1:2, and with a totalthickness in the range of about 10 nm to about 1.5 μm, such as fromabout 10 nm to about 1 um or from about 10 nm to about 300 nm. Inparticular, BY₂ and AX can be mixed in an evaporator boat and thendeposited by thermal evaporation. Mixing of BY₂ and AX can be carriedout in a powdered form, or by forming a pre-melt of BY₂ and AX. In thecase of SnI₂ (or SnCl₂) and CsI, SnI₂ (or SnCl₂) can evaporate at lowertemperatures than CsI, and, therefore, a temperature of the evaporatorboat can be gradually raised as a relative amount of CsI in a mixtureincreases.

Different types of source(B) can be used, and can be co-deposited withsource(A, X) or deposited sequentially with source(A, X). For example,BY₂ and B′Y′₂ can be mixed in an evaporator boat and deposited bythermal evaporation, followed by deposition of AX, and so forth. Mixingof BY₂ and B′Y′₂ can be carried out in a powdered form, or by forming apre-melt of BY₂ and B′Y′₂, with a weight or molar ratio of BY₂ to B′Y′₂from about 99:1 to about 1:99, such as from about 5:1 to about 1:5 orfrom about 2:1 to about 1:2. As another example, BY₂, B′Y′₂, and B″Y″₂can be mixed in an evaporator boat and deposited by thermal evaporation,followed by deposition of AX, and so forth. Likewise, different types ofsource(A, X) can be used, and can be co-deposited with source(B) ordeposited sequentially with source(B).

Still referring to FIG. 5, an encapsulation material is next deposited,thereby forming an encapsulation layer 504 adjacent to the precursorlayer 502. The encapsulation layer 504 serves to provide protection andsealing of a resulting luminescent material and to reduce its exposureto oxygen, humidity, and other contaminants, thereby enhancing stabilityof resulting photoluminescence characteristics. Examples of suitableencapsulation materials include oxides, such as silica, alumina, TiO₂,Ta₂O₅, Nb₂O₅, ZrO₂, HfO₂, SnO₂, ZnO₂, La₂O₃, Y₂O₃, CeO₂, Sc₂O₃, Er₂O₃,V₂O₅, and In₂O₃; nitrides, such as SiO_(x)N_(2-x); fluorides, such asCaF₂, SrF₂, ZnF₂, MgF₂, LaF₃, and GdF₂; nanolaminates, such asHfO₂/Ta₂O₅, TiO₂/Ta₂O₅, TiO₂/Al₂O₃, ZnS/Al₂O₃, and AITiO; and othersuitable thin-film dielectric materials. Additional examples of suitableencapsulation materials include glasses, such as borosilicate glasses(e.g., borofloat® glass and D 263™ glass), phosphate glasses, and otherlow melting glasses. A thickness of the encapsulation layer 504 can bein the range of about 10 nm to about 1.5 μm, such as from about 50 nm toabout 500 nm or from about 50 nm to about 300 nm. In the case of adeposited layer of glass, a suitable metal can be deposited adjacent tothe layer of glass to provide a hermetic seal, such as silver, aluminum,gold, copper, iron, cobalt, nickel, palladium, platinum, ruthenium, oriridium. Vacuum deposition, such as electron-beam evaporation, can beused to form the encapsulation layer 504, along with the other layers ofthe assembly of layers. Alternatively, another suitable depositiontechnique can be used, such as sputtering.

The assembly of layers is next subjected to annealing to promote bondingbetween the layers as well as to promote reaction according to formula(48), thereby converting the precursor layer 502 to an emission layerincluding a luminescent material according to formula (1). Annealing canbe carried out using any suitable heating technique to apply thermalenergy via conduction, convection, or radiation heating, such as byheating the assembly of layers using a hot plate, an oven, resistheating, or lamp heating. It is also contemplated that thermal energycan be applied in accordance with fast heating cycles to yield rapidthermal annealing.

Resulting photoluminescence characteristics can be dependent upon anannealing temperature and an annealing time period. As such, anannealing temperature and an annealing time period can be optimized toyield higher photoluminescence intensities. For example, a particularone of BY₂ and AX can have a lower melting point T_(m1), another one ofBY₂ and AX can have a higher melting point T_(m2), and an optimalannealing temperature T_(heat) can be greater than T_(m1) and less thanT_(m2), such as greater than T_(m1) and up to a three-quarters point(i.e., (T_(m1)+3T_(m2))/4) or a halfway point (i.e., (T_(m1)+T_(m2))/2)between the lower melting point and the higher melting point, althoughannealing can also be carried out at higher or lower temperatures. Inthe case of SnI₂ with a melting point of about 318° C. and CsI with amelting point of about 620° C., an optimal annealing temperatureT_(heat) can be greater than about 318° C. and less than about 620° C.,such as from about 340° C. to about 420° C. or from about 350° C. toabout 410° C. In the case of SnCl₂ with a melting point of about 246° C.and CsI with a melting point of about 620° C., an optimal annealingtemperature T_(heat) can be greater than about 246° C. and less thanabout 620° C. In some instances, an initial melting can arise fromformation of an eutectic between SnCl₂ and a reaction product of SnCl₂and CsI. An optimal annealing time period can be in the range of about 1sec to about 1 hr, such as from about 5 sec to about 10 min or fromabout 5 sec to about 1 min, although annealing can also be carried outfor longer or shorter time periods. Optimal values of an annealingtemperature and an annealing time period can also be suitably adjusteddepending upon, for example, particular reagents used, a thickness ofindividual layers within the precursor layer 502, or a total thicknessof the precursor layer 502. In some instances, a reaction between layersof reactants can occur at temperatures significantly below meltingtemperatures of the reactants by way of solid state reactions. Inparticular, the layers can be sufficiently thin so that diffusion canoccur within, for example, a few hundred nanometers or less and a timeperiod of a few seconds to a few minutes, thereby allowing the reactantsto react and to form a luminescent material.

Referring next to FIG. 6, a substrate 600 is provided, and a precursorlayer 604 is formed adjacent to the substrate 600. Certain aspects ofthe manufacturing method of FIG. 6 can be implemented in a similarmanner as described above for FIG. 5, and, therefore, are not furtherdescribed herein.

As illustrated in FIG. 6, a reflector layer 602 is initially formedadjacent to the substrate 600, followed by formation of the precursorlayer 604 adjacent to the reflector layer 602, and followed by formationof another reflector layer 606 adjacent to the precursor layer 604. Thepair of reflector layers 602 and 606 serve to provide protection andsealing of a resulting luminescent material and to reduce its exposureto oxygen, humidity, and other contaminants. In addition, the pair ofreflector layers 602 and 606 serve to reduce loss of emitted radiationand to promote guiding of the emitted radiation along a lateraldirection.

In the illustrated embodiment, the formation of the reflector layers 602and 606 is carried out by depositing a set of dielectric materials usingALD or another suitable deposition technique. In particular, each of thereflector layers 602 and 606 is implemented as a dielectric stack,including multiple dielectric layers and with the number of dielectriclayers in the range of 2 to 1,000, such as in the range of 2 to 100, inthe range of 30 to 90, or in the range of 30 to 80. Each individualdielectric layer can have a thickness in the range of about 1 nm toabout 200 nm, such as from about 10 nm to about 150 nm or from about 10nm to about 100 μm. Depending upon the number of dielectric layersforming the reflector layers 602 and 606, a total thickness of each ofthe reflector layers 602 and 606 can be in the range of about 100 nm toabout 20 μm, such as from about 1 μm to about 15 μm or from about 1 μmto about 10 μm. For certain implementations, a dielectric stack caninclude multiple layers formed from different dielectric materials.Layers formed from different materials can be arranged in a periodicfashion, such as in an alternating fashion, or in a non-periodicfashion. Examples of dielectric materials that can be used to form thereflector layers 602 and 606 include oxides, such as silica, alumina,TiO₂, Ta₂O₅, Nb₂O₅, ZrO₂, HfO₂, SnO₂, ZnO₂, La₂O₃, Y₂O₃, CeO₂, Sc₂O₃,Er₂O₃, V₂O₅, and In₂O₃; nitrides, such as SiO_(x)N_(2-x); fluorides,such as CaF₂, SrF₂, ZnF₂, MgF₂, LaF₃, and GdF₂; nanolaminates, such asHfO₂/Ta₂O₅, TiO₂/Ta₂O₅, TiO₂/Al₂O₃, ZnS/Al₂O₃, and AlTiO; and othersuitable thin-film dielectric materials. Desirably, different materialsforming a dielectric stack have different refractive indices so as toform a set of high index layers and a set of low index layers that areinterspersed within the dielectric stack. For certain implementations,an index contrast in the range of about 0.3 to about 1 or in the rangeof about 0.3 to about 2 can be desirable. For example, TiO₂ and SiO₂ canbe included in alternating layers of a dielectric stack to provide arelatively large index contrast between the layers. A larger indexcontrast can yield a larger stop band with respect to emitted radiation,thereby approaching the performance of an ideal omnireflector. Inaddition, a larger index contrast can yield a greater angular tolerancefor reflectivity with respect to incident solar radiation, and canreduce a leakage of emitted radiation at larger angles from a normaldirection.

The assembly of layers illustrated in FIG. 6 is then subjected toannealing to promote bonding between the layers as well as to promotereaction according to formula (48), thereby converting the precursorlayer 604 to an emission layer that is sandwiched between the reflectorlayers 602 and 606.

Referring next to FIG. 7, a reflector layer 702 is formed adjacent to asubstrate 700, and a precursor layer 704 is formed adjacent to thereflector layer 702. Certain aspects of the manufacturing method of FIG.7 can be implemented in a similar manner as described above for FIG. 5and FIG. 6, and, therefore, are not further described herein.

As illustrated in FIG. 7, a spacer layer 706 is next formed adjacent tothe precursor layer 704, followed by formation of another reflectorlayer 708 adjacent to the spacer layer 706. The spacer layer 706 and thereflector layer 708 serve to provide protection and sealing of aresulting luminescent material and to reduce its exposure to oxygen,humidity, and other contaminants. In addition, the reflector layer 708serves to reduce loss of emitted radiation and to promote guiding of theemitted radiation along a lateral direction, while the spacer layer 706provides index matching for low loss guiding of the emitted radiation.It is contemplated that the relative positions of the reflector layer702 and the reflector layer 708, with respect to the precursor layer704, can be switched for other implementations, and that the spacerlayer 706 (or another similar spacer layer) and the reflector layer 708(or another similar reflector layer) can be formed adjacent to thesubstrate 700, with the spacer layer 706 (or another similar spacerlayer) providing a deposition surface for the formation of the precursorlayer 704.

In the illustrated embodiment, the formation of the spacer layer 706 andthe reflector layer 708 is carried out by depositing a set of materialsusing vacuum deposition or another suitable deposition technique. Inparticular, the reflector layer 708 is implemented so as to beomnireflective over a relatively wide range of wavelengths, and can beformed by depositing a metal, such as silver, aluminum, gold, copper,iron, cobalt, nickel, palladium, platinum, ruthenium, or iridium; ametal alloy; or another suitable material having broadband reflectivity.A thickness of the reflector layer 708 can be in the range of about 1 nmto about 200 nm, such as from about 10 nm to about 150 nm or from about10 nm to about 100 μm. As illustrated in FIG. 7, the spacer layer 706can be formed by depositing a suitable low index material, such onehaving a refractive index that is no greater than about 2, no greaterthan about 1.5, or no greater than about 1.4. Examples of suitable lowindex materials include MgF₂ (refractive index of about 1.37), CaF₂,silica, alumina, and other suitable thin-film, low index, dielectricmaterials. A thickness of the spacer layer 706 can be in the range ofabout 1 nm to about 500 nm, such as from about 50 nm to about 300 nm orfrom about 10 nm to about 100 nm.

The assembly of layers illustrated in FIG. 7 is then subjected toannealing to promote bonding between the layers as well as to promotereaction according to formula (48), thereby converting the precursorlayer 704 to an emission layer that is sandwiched between the reflectorlayers 702 and 708.

Referring next to FIG. 8, a bottom substrate 800 is provided, and aprecursor layer 802 is formed adjacent to the bottom substrate 800.While not illustrated in FIG. 8, it is contemplated that either, orboth, a spacer layer and a reflector layer can be formed adjacent to thebottom substrate 800 and can provide a deposition surface for theformation of the precursor layer 802. Certain aspects of themanufacturing method of FIG. 8 can be implemented in a similar manner asdescribed above for FIG. 5 through FIG. 7, and, therefore, are notfurther described herein.

As illustrated in FIG. 8, a bonding layer 804 is next formed adjacent tothe precursor layer 802, followed by positioning of a top substrate 806adjacent to the bonding layer 804, and followed by bonding and annealingof the assembly of layers. The bonding layer 804 and the top substrate806 serve to provide protection and sealing of a resulting luminescentmaterial and to reduce its exposure to oxygen, humidity, and othercontaminants. It is contemplated that the bonding layer 804 (or anothersimilar bonding layer) can be formed adjacent to the top substrate 806,and then positioned adjacent to the precursor layer 802 to achievebonding.

In the illustrated embodiment, the formation of the bonding layer 804 iscarried out by depositing an adhesive or bonding material using anysuitable deposition technique. In particular, the bonding layer 804 canbe formed by depositing a thermal-curable adhesive or bonding material,such as a glass (e.g., a spin-on glass or a sealing glass) or a polymer(e.g., a perfluoropolymer or an epoxy-based polymer). A thickness of thebonding layer 804 can be in the range of about 1 nm to about 200 nm,such as from about 10 nm to about 150 nm or from about 10 nm to about100 μm.

Bonding can be achieved using fluid pressure, a mechanical press, oranother suitable bonding technique, along with the application ofthermal energy to promote bonding as well as to promote reactionaccording to formula (48). It is also contemplated that bonding can beachieved using an ultraviolet light-curable adhesive or bondingmaterial, rather than a thermal-curable adhesive or bonding material.For example, a thin coating of a pre-polymer (or a set of monomers) canbe applied to a set of surfaces, and, after the surfaces are pressedtogether, ultraviolet light exposure can be applied through either, orboth, of the surfaces to cure the pre-polymer (or the monomers).

Solar Modules

FIG. 9 illustrates a solar module 900 implemented in accordance with anembodiment of the invention. The solar module 900 includes a PV cell902, which is a p-n junction device formed from crystalline silicon.However, the PV cell 902 can also be formed from another suitablephotoactive material. As illustrated in FIG. 9, the PV cell 902 isimplemented as a thin slice or strip of crystalline silicon. The use ofthin slices of silicon allows a reduction in silicon consumption, which,in turn, allows a reduction in manufacturing costs. Micromachiningoperations can be performed on a silicon wafer to form numerous siliconslices, and each of the silicon slices can be further processed to formPV cells, such as the PV cell 902. As illustrated in FIG. 9, the PV cell902 is configured to accept and absorb radiation incident upon a sidesurface 904 of the PV cell 902, although other surfaces of the PV cell902 can also be involved.

In the illustrated embodiment, the solar module 900 also includes aspectral concentrator 906, which is formed as a slab having a sidesurface 908 that is adjacent to the side surface 904 of the PV cell 902.The spectral concentrator 906 includes a set of luminescent materials asdescribed herein, which can be included within an emission layer toconvert a relatively wide range of energies of solar radiation into arelatively narrow, substantially monochromatic energy band that ismatched to an absorption spectrum of the PV cell 902. During operationof the solar module 900, incident solar radiation strikes a top surface910 of the spectral concentrator 906, and a certain fraction of thisincident solar radiation penetrates below the top surface 910 and isabsorbed and converted into substantially monochromatic, emittedradiation. This emitted radiation is guided laterally within thespectral concentrator 906, and a certain fraction of this emittedradiation reaches the side surface 904 of the PV cell 902, which absorbsand converts this emitted radiation into electricity.

In effect, the spectral concentrator 906 performs a set of operations,including: (1) collecting incident solar radiation; (2) converting theincident solar radiation into substantially monochromatic, emittedradiation near a bandgap energy of the PV cell 902; and (3) conveyingthe emitted radiation to the PV cell 902, where the emitted radiationcan be converted to useful electrical energy. The spectral concentrator906 can include distinct structures that are optimized or otherwisetailored towards respective ones of the collection, conversion, andconveyance operations. Alternatively, certain of these operations can beimplemented within a common structure. These operations that areperformed by the spectral concentrator 906 are further described below.

Collection refers to capturing or intercepting incident solar radiationin preparation for conversion to emitted radiation. Collectionefficiency of the spectral concentrator 906 can depend upon the amountand distribution of a luminescent material within the spectralconcentrator 906. In some instances, the luminescent material can beviewed as a set of luminescent centers that can intercept incident solarradiation, and a greater number of luminescent centers typicallyincreases the collection efficiency. Depending upon the distribution ofthe luminescent centers, collection of incident solar radiation canoccur in a distributed fashion throughout the spectral concentrator 906,or can occur within one or more regions of the spectral concentrator906. The collection efficiency can also depend upon other aspects of thespectral concentrator 906, including the ability of incident solarradiation to reach the luminescent material. In particular, thecollection efficiency is typically improved by suitable optical couplingof incident solar radiation to the luminescent material, such as via ananti-reflection coating to reduce reflection of incident solarradiation.

Conversion refers to emitting radiation in response to incident solarradiation, and the efficiency of such conversion refers to theprobability that an absorbed solar photon is converted into an emittedphoton. Conversion efficiency of the spectral concentrator 906 candepend upon photoluminescence characteristics of a luminescent material,including its optical quantum efficiency or its internal quantumefficiency, but can also depend upon interaction of luminescent centerswith their local optical environment, including via resonant cavityeffects. Depending upon the distribution of the luminescent centers,conversion of incident solar radiation can occur in a distributedfashion throughout the spectral concentrator 906, or can occur withinone or more regions of the spectral concentrator 906. Also, dependingupon the particular luminescent material used, the conversion efficiencycan depend upon wavelengths of incident solar radiation that areabsorbed by the luminescent material.

Conveyance refers to guiding or propagation of emitted radiation towardsthe PV cell 902, and the efficiency of such conveyance refers to theprobability that an emitted photon reaches the PV cell 902. Conveyanceefficiency of the spectral concentrator 906 can depend uponphotoluminescence characteristics of a luminescent material, including adegree of overlap between emission and absorption spectra, but can alsodepend upon interaction of luminescent centers with their local opticalenvironment, including via resonant cavity effects.

By performing these operations, the spectral concentrator 906 provides anumber of benefits. In particular, by performing the collectionoperation in place of the PV cell 902, the spectral concentrator 906allows a significant reduction in silicon consumption, which, in turn,allows a significant reduction in manufacturing costs. In someinstances, the amount of silicon consumption can be reduced by a factorof about 10 to about 1,000. Also, the spectral concentrator 906 enhancessolar energy conversion efficiency based on at least two effects: (1)concentration effect; and (2) monochromatic effect.

In terms of the concentration effect, the spectral concentrator 906performs spectral concentration by converting a relatively wide range ofenergies of incident solar radiation into a narrow band of energiesclose to the bandgap energy of the PV cell 902. Incident solar radiationis collected via the top surface 910 of the spectral concentrator 906,and emitted radiation is guided towards the side surface 904 of the PVcell 902. A solar radiation collection area, as represented by, forexample, an area of the top surface 910 of the spectral concentrator906, can be significantly greater than an area of the PV cell 902, asrepresented by, for example, an area of the side surface 904 of the PVcell 902. A resulting concentration factor onto the PV cell 902 can bein the range of about 10 to about 100 and up to about 1,000 or more. Forexample, the concentration factor can exceed about 10,000 and can be upto about 60,000 or more. In turn, the concentration factor can increasethe open circuit voltage or V_(oc) of the solar module 900, and canyield an increase in solar energy conversion efficiency of about 2percent (absolute), or 10 percent (relative), for each concentrationfactor of 10 in emitted radiation reaching the PV cell 902. For example,V_(oc) can be increased from a typical value of about 0.55 V, which isabout half the bandgap energy of silicon, to about 1.6 V, which is about1.5 times the bandgap energy of silicon. A typical solar radiationenergy flux or intensity is about 100 mW cm⁻², and, in some instances, aconcentration factor of up to 10⁶ (or more) can be achieved byoptimizing the spectral concentrator 906 with respect to the collection,conversion, and conveyance operations.

In terms of the monochromatic effect, the narrow band radiation emittedfrom the spectral concentrator 906 can be efficiently absorbed by the PVcell 902, which can be optimized in terms of its junction design tooperate on this narrow band, emitted radiation. In addition, by matchingthe energy of the emitted radiation with the bandgap energy of the PVcell 902, thermalization can mostly occur within the spectralconcentrator 906, rather than within the PV cell 902.

Aspects of Cavity Quantum Electrodynamics can be used to implement thespectral concentrator 906 as a micro-cavity or a resonant cavitywaveguide. The resulting resonant cavity effects can provide a number ofbenefits. For example, resonant cavity effects can be exploited tocontrol a direction of emitted radiation towards the PV cell 902 and,therefore, enhance the fraction of emitted radiation reaching the PVcell 902. This directional control can involve suppressing emission foroptical modes in non-guided directions, while allowing or enhancingemission for optical modes in guided directions towards the PV cell 902.In such manner, there can be a significant reduction in loss of emittedradiation via a loss cone. Also, resonant cavity effects can beexploited to modify emission characteristics, such as by enhancingemission of a set of wavelengths that are associated with certainoptical modes and suppressing emission of another set of wavelengthsthat are associated with other optical modes. This modification ofemission characteristics can reduce an overlap between an emissionspectrum and an absorption spectrum via spectral pulling, and can reducelosses arising from self-absorption. This modification of emissioncharacteristics can also yield a larger exciton binding energy, and canpromote luminescence via exciton emission. In addition, resonant cavityeffects can enhance absorption and emission characteristics of a set ofluminescent materials, and can allow the use of semiconductor materialshaving indirect optical transitions or forbidden optical transitions.This enhancement of absorption and emission characteristics can involveoptical gain as well as amplified spontaneous emission, such as via thePurcell effect. In some instances, the high intensity of emittedradiation within the spectral concentrator 906 can lead to stimulatedemission and lasing, which can further reduce losses as emittedradiation is guided towards the PV cell 902.

In the illustrated embodiment, a local density of optical states withinthe spectral concentrator 906 can include both guided optical modes andradiative optical modes. Guided optical modes can involve propagation ofemitted radiation along the spectral concentrator 906, while radiativeoptical modes can involve propagation of emitted radiation out of thespectral concentrator 906. For a relatively low degree of verticalconfinement, the local density of optical states and emissioncharacteristics are modified to a relatively low degree. Increasingvertical confinement, such as by increasing an index contrast betweendielectric layers of a dielectric stack, can introduce greaterdistortions in the local density of optical states, yieldingmodification of emission characteristics including directional control.Also, by adjusting a thickness of an emission layer within the spectralconcentrator 906 with respect to vertical resonance, radiative opticalmodes can be suppressed. This suppression can reduce emission losses outof the spectral concentrator 906, while enhancing probability of lateralemission along the spectral concentrator 906 in a direction towards thePV cell 902. For certain implementations, the emission layer can bedisposed between a pair of reflector layers so as to be substantiallycentered at an anti-node position of a resonant electromagnetic wave,and the pair of reflector layers can be spaced to yield a cavity lengthin the range of a fraction of a wavelength to about ten wavelengths ormore. Lateral confinement can also be achieved by, for example, formingreflector layers adjacent to side edges and surfaces of the spectralconcentrator 906, which are not involved in conveyance of radiation.

When implemented as a resonant cavity waveguide, a performance of thespectral concentrator 906 can be characterized with reference to itsquality or Q value, which can vary from low to high. A relatively low Qvalue can be sufficient to yield improvements in efficiency, with agreater Q value yielding additional improvements in efficiency. Forcertain implementations, the spectral concentrator 906 can have a Qvalue that is at least about 5, such as at least about 10 or at leastabout 100, and up to about 10⁵ or more, such as up to about 10,000 or upto about 1,000. In the case of a high-Q resonant cavity waveguide, thespectral concentrator 906 can exhibit an exciton emission in whichexcitons interact with cavity photons to form coupled exciton-photonquasi-particles referred as exciton-polaritons. The spectralconcentrator 906 can operate in a weak coupling regime or a strongcoupling regime, depending upon an extent of coupling between excitonsand cavity photons or among excitons in the case of bi-excitons.

In the strong coupling regime, the spectral concentrator 906 can beimplemented as a polariton laser, which can lead to highly efficient andintense emissions and extremely low lasing thresholds. A polariton lasercan have substantially zero losses and an efficiency up to about 100percent. A polariton laser is also sometimes referred as a zerothreshold laser, in which there is little or no lasing threshold, andlasing derives at least partly from excitons or related quasi-particles,such as bi-excitons or exciton-polaritons. The formation ofquasi-particles and a resulting modification of energy levels or statescan reduce losses arising from self-absorption. Contrary to conventionallasers, a polariton laser can emit coherent and substantiallymonochromatic radiation without population inversion. Without beingbound by a particular theory, emission characteristics of a polaritonlaser can occur when exciton-polaritons undergo Bose-condensation withina resonant cavity waveguide. Lasing can also occur in the weak couplingregime, although a lasing threshold can be higher than for the strongcoupling regime. In the weak coupling regime, lasing can deriveprimarily from excitons, rather than from exciton-polaritons.

By implementing as a high-Q resonant cavity waveguide in the form of apolariton laser, the spectral concentrator 906 can exhibit a number ofdesirable characteristics. In particular, lasing can be achieved with avery low threshold, such as with an excitation intensity that is nogreater than about 200 mW cm⁻², no greater than about 100 mW cm⁻², nogreater than about 50 mW cm⁻², or no greater than about 10 mW cm⁻², anddown to about 1 mW cm⁻² or less, which is several orders of magnitudesmaller than for a conventional laser. Because a typical solar radiationintensity is about 100 mW cm⁻², lasing can be achieved with normalsunlight with little or no concentration. Also, lasing can occur with ashort radiative lifetime, such as no greater than about 500 psec, nogreater than about 200 psec, no greater than about 100 psec, or nogreater than about 50 psec, and down to about 1 psec or less, which canavoid or reduce relaxation through non-radiative mechanisms.Furthermore, lasing can involve narrowing of a spectral width of anemission spectrum to form a narrow emission line, such as by a factor ofat least about 1.5, at least about 2, or at least about 5, and up toabout 10 or more, relative to the case where there is a substantialabsence of resonant cavity effects. For example, in the case of UD930, aspectral width can be narrowed to a value in the range of about 2 nm toabout 10 nm, such as from about 3 nm to about 10 nm, when UD930 isincorporated in a high-Q resonant cavity waveguide. A narrow emissionline from lasing can enhance solar conversion efficiencies, as a resultof the monochromatic effect. In such manner, lasing and low loss withdistance can allow higher intensities of emissions reaching the PV cell902 and higher solar conversion efficiencies. There can be little or nomeasurable loss of emissions that are guided towards the PV cell 902.With lasing, a solar energy conversion efficiency can be up to about 30percent or more, such as in the range of about 20 percent to about 30percent or in the range of about 28 percent to about 30 percent.

EXAMPLES

The following examples describe specific aspects of some embodiments ofthe invention to illustrate and provide a description for those ofordinary skill in the art. The examples should not be construed aslimiting the invention, as the examples merely provide specificmethodology useful in understanding and practicing some embodiments ofthe invention.

Example 1 Formation of Luminescent Material—UD930

Samples of UD930 were formed in a reproducible manner by vacuumdeposition in accordance with two approaches. In accordance with oneapproach, tin(II) iodide and cesium(I) iodide were evaporated insequential layers, typically up to six layers total. In accordance withanother approach, tin(II) chloride and cesium(I) iodide were evaporatedin sequential layers, typically up to six layers total. For certainsamples, a thickness of the tin(II) chloride-containing layer was about90 nm, and a thickness of the cesium(I) iodide-containing layer wasabout 170 nm. Tin(II) iodide (or tin(II) chloride) was deposited bythermal evaporation, while cesium(I) iodide was deposited byelectron-beam evaporation. Deposition was carried out at a pressure ofabout 10⁻⁵ Torr (or less) on a variety of substrates, including thoseformed from glass, ceramic, and silicon.

Following deposition, resulting samples were annealed in a glove box ina dry, nitrogen atmosphere to allow a self-limiting chemical reactionbetween tin(II) iodide (or tin(II) chloride) and cesium(I) iodide. Inthe case of samples formed using tin(II) chloride, annealing was alsosometimes carried out in air. An optimum annealing temperature wasobserved to vary somewhat depending upon a particular sample, buttypically was greater than a melting point of tin(II) iodide at about318° C. (or a melting point of tin(II) chloride at about 246° C.). FIG.10 illustrates measured photoluminescence intensity at about 943 nmplotted as a function of annealing temperature for tin(II)iodide/cesium(I) iodide deposited on a silicon substrate (with nativeSiO₂), according to an embodiment of the invention. Annealing attemperatures at or below the melting point of tin(II) iodide wasobserved to yield weak photoluminescence intensity, while a strong bandof photoluminescence intensities was observed after annealing attemperatures of about 50° C. above the melting point of tin(II) iodide.An optimum annealing time period was observed to vary somewhat dependingupon a particular sample and a particular substrate used. In the case ofsamples formed on glass substrates, an optimal annealing time periodtypically was about 15 sec. Stability of photoluminescence was enhancedif the samples were stored in a dry, nitrogen atmosphere, or wereencapsulated, such as by bonding to a glass substrate using a layer ofepoxy.

Example 2 Characterization of Luminescent Material—UD930

FIG. 11( a) illustrates excitation spectra obtained for UD930 attemperatures in the range of 12K to 300K, and FIG. 11( b) illustratesemission spectra obtained for UD930 in the same temperature range,according to an embodiment of the invention. As can be appreciated, avariation of the peak positions with temperature is similar for theexcitation and emission spectra.

A practitioner of ordinary skill in the art requires no additionalexplanation in developing the luminescent materials described herein butmay nevertheless find some helpful guidance by examining the co-pendingand co-owned U.S. patent application Ser. No. 11/689,381 (now U.S. Pat.No. 7,641,815), filed on Mar. 21, 2007, the disclosure of which isincorporated herein by reference in its entirety.

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

1. A luminescent material having the formula:[A_(a)Sn_(b)X_(x)X′_(x′)X″_(x″)][dopant], wherein A is included in theluminescent material as a monovalent cation; X, X′, and X″ are selectedfrom fluorine, chlorine, bromine, and iodine; a is in the range of 1 to5; b is in the range of 1 to 3; a sum of x, x′, and x″ is a+2 b; and atleast X′ is iodine, such that x′/(a+2b)≧⅕.
 2. The luminescent materialof claim 1, wherein A is selected from potassium, rubidium, and cesium.3. The luminescent material of claim 1, wherein A is cesium, and a is 1or
 2. 4. The luminescent material of claim 1, wherein b is 1 or
 2. 5.The luminescent material of claim 1, wherein at least one of X and X″ isfluorine.
 6. The luminescent material of claim 1, wherein at least oneof X and X″ is chlorine.
 7. The luminescent material of claim 1, whereinat least one of X and X″ is bromine.
 8. The luminescent material ofclaim 1, wherein x′/(a+2b)≧½.
 9. The luminescent material of claim 1,wherein x′/(a+2b)≧⅔.
 10. The luminescent material of claim 1, wherein ais 1, and b is
 1. 11. The luminescent material of claim 1, wherein a is1, and b is
 2. 12. The luminescent material of claim 1, wherein a is 2,and b is
 1. 13. The luminescent material of claim 1, wherein the dopantis included in the luminescent material in an amount less than 5 percentin terms of elemental composition.
 14. The luminescent material of claim1, wherein the dopant is included in the luminescent material in anamount in the range of 0.1 percent to 1 percent in terms of elementalcomposition.
 15. The luminescent material of claim 1, wherein the dopantincludes anions including oxygen.
 16. The luminescent material of claim14, wherein the anions include at least one of O⁻² and OH⁻¹.
 17. Amethod of forming a luminescent material, comprising: providing a sourceof A and X, wherein A is selected from at least one of elements of GroupIA, and X is selected from at least one of elements of Group VIIB;providing a source of B, wherein B is selected from at least one ofelements of Group IVB; subjecting the source of A and X and the sourceof B to vacuum deposition to form a set of films adjacent to asubstrate; and heating the set of films to a temperature T_(heat) toform a luminescent material adjacent to the substrate, wherein theluminescent material includes A, B, and X, one of the source of A and Xand the source of B has a lower melting point T_(m1), another of thesource of A and X and the source of B has a higher melting point T_(m2),and T_(m1)<T_(heat)<T_(m2).
 18. The method of claim 17, whereinT_(m1)<T_(heat)<(T_(m1)+3T_(m2))/4.
 19. The method of claim 17, whereinT_(m1)<T_(heat)<(T_(m1)+T_(m2))/2.
 20. The method of claim 17, whereinthe source of A and X includes a compound having the formula AX, and thesource of B includes a compound having the formula BY₂, where Y isselected from at least one of elements of Group VIIB.
 21. The method ofclaim 17, wherein subjecting the source of A and X and the source of Bto vacuum deposition includes: subjecting the source of A and X to atleast one of electron-beam deposition and thermal evaporation; andsubjecting the source of B to thermal evaporation.
 22. The method ofclaim 17, wherein subjecting the source of A and X and the source of Bto vacuum deposition includes: mixing the source of A and X and thesource of B to form a mixture; and subjecting the mixture to vacuumdeposition to form the set of films adjacent to the substrate.